© Applied Science Innovations Pvt. Ltd., India Carbon Sci ......2019/03/11 · Magnetometer (VSM)...

© Applied Science Innovations Pvt. Ltd., India Carbon – Sci. Tech. 11/3 (2019) 28-40

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RESEARCH ARTICLE Received: 21/06/2019; Accepted: 05/07/2019

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Coconut oil capped nano iron oxide for EMI shielding application

Arya M. A (a), Madhvi Tiwari (1), Priyesh V. More (1), Saurabh Parmar (2), Suwarna Datar (2) and

Pawan K. Khanna (1, *)

(a) Department of Applied Chemistry, Defence Institute of Advanced Technology (DIAT), DRDO, Pune

- 411025, Maharashtra, India.

(b) Department of Applied Physics, Defence Institute of Advanced Technology, DRDO, Pune - 411025,

Maharashtra, India.

Abstract: The super-paramagnetic particles of large surface area with controlled size Fe2O3

nanoparticles are synthesized by addition of reducing agents along with green capping agents via Co-

precipitation method which is one of the simplest and productive method to synthesize such nano-

particles in large quantities. γ-Fe2O3 NPs so-obtained as confirmed by XRD were further characterized

by various analytical techniques like UV-Visible, and FT-IR Spectroscopy, SEM/EDX and PSA.

Nanocomposite films of very small thickness are fabricated by blending capped and uncapped Fe2O3

nanoparticles in PVA matrix at different loading concentration and their shielding efficiency is studied

using Vector Network Analyzer (VNA) in the X-band frequency (8.2-12.4 GHz) and Ku-band frequency

(12.4-18 GHz). EMI SE in Ku-band was maximum for capped Fe2O3 NPs PVA film with value of -13.06

dB, better than uncapped Fe2O3 NPs/PVA film.

Key words: Green surfactant, nano-particles, polymer composites, EMI shielding ------------------------------------------------------------------------------------------------------------------------------------------------

1. Introduction: Recently magnetic nanoparticles are in great demand due to their unique properties

such as paramagnetic behavior, have highly magnetic sensitivity, high coerciveness etc. They are

preferred in a very wide range of fields including magnetic fluids, data storage techniques, catalysis, and

biomedical applications [1]. In the last decade, several types of iron oxide nanoparticles were

investigated viz; magnetite (Fe3O4), ferromagnetic and/or superparamagnetic at <15 nm (FeIIFeIII2O4),

hematite (α-Fe2O3), maghemite (γ-Fe2O3), wϋstite, antiferromagnetic (FeO), ԑ-Fe2O3 and β-Fe2O3. Such

nano-particles due to exposure to air, can get oxidized resulting in loss of magnetism and dispersability.

A comparison between two important iron oxide nanoparticles, maghemite (γ-Fe2O3) and magnetite

(Fe3O4) has been well documented [2]. Size, shape, surface chemistry and the application of nanoparticle

is dependent on the preparation method and thus has always been a challenging task for chemists and

materials scientists. A large variety of chemical route have attracted researchers e.g. co-precipitation,

microemulsion, hydrothermal, sonochemical, and thermal decomposition etc. Co-precipitation is the one

of the most popular methods in terms of economics and commercial production [2-4]. Despite several

advantages that co-precipitation suffers by often compromising the particles size distribution. To,

overcome such drawbacks many research groups reported modifications and improvements. Ahn et al

[3] suggested the stoichiometric ratio of iron salts Fe2+, Fe3+ as 1:2 with the addition of ammonia

solution [NH3 (aq)] by varying pH from 1.5 to 11.0 at 25˚C. Kim et al [5] proposed synthesis of

superparamagnetic monodispersed iron oxide NPs by co-precipitation with ferric chloride hexahydrate

under N2 atmosphere. Morales [6] differentiated between α-Fe2O3 and γ-Fe2O3 using the co-precipitation

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method and single precursor of Fe (III) salt and drop wise addition of NaOH solution. Lee et al [7]

introduced another method called piezoelectric nozzle method for synthesizing maghemite particles

where the reducing agent hydrochloric acid (HCl) is added through pipette nozzle to prepare particles of

size 3 to 8 nm. The above methods preferred only conducting the reaction in nitrogen atmosphere to

obtain only maghemite particles, in alkaline pH and high temperature in the range of 75 ˚C – 90 ˚C. The

introduction of capping agent further controls the size of particles. Morales et al [6] carried out the

reaction using polyvinyl alcohol (PVA) as surfactant to obtain smaller particles. Budhwar et al [8]

employed coconut oil as surfactant by adapting reported method of co-precipitation and their

modification particles with surface area of 45.65 m2/g can be synthesized easily. The availability of

various size, shaped and morphology of iron oxide, wide application potential has been documented in

electrical, magnetic and electronic devices. Iron oxide has special place of application when it comes to

electronic devices including safety from electromagnetic radiation.

In the era of technology, humans are connected deeply with modern communication devices such as

mobile phones, radios, laptops, television etc. With the ever increasing use and number of electronic

devices in almost all areas that human might cover, the harmful effects of also increases due radiation.

All the electronic devices either absorb or radiate electromagnetic (EM) waves and affect the

performance of other devices or electronic circuits thereby creates interference [9]. Thus EM

interference (EMI) has become one of the major issues. Multifunctional and light weight EMI shielding

composite materials are required to minimize EM interference to offer better shield for humans to avoid

health hazards.

Based on the understanding from the literature, it is realized that the research is mainly focused in

developing EMI shielding in both X (8 – 12 GHz) and Ku-bands (12 - 18 GHz), for applications in a

variety of devices/fields. Conventionally many metal based shields were used due to their firmness and

corrodibility but they were eventually substituted by polymers for want of flexible, lightweight, anti-

corrosive and cost-effective materials. Polymers are being widely used in many applications including

sensors, capacitors, circuits, batteries etc. Amongst them Polyvinyl alcohol (PVA) and Poly (methyl

methacrylate) (PMMA) are the most preferred non-conducting polymers due to their due to their high

breakdown strength, reproducibility, popularity and versatility. PVA is often more preferred because it

does not react with other chemicals, provides a flexibility and durability while casting and is compatible

with wide range of fillers in addition to its ability to tolerate reasonable temperature range. An effective

EMI shield is dominated by three main functions: reflection, absorption and multiple internal reflections.

On the surface of shielding materials usually reflection occurs because the incoming EM waves are

interacting with surface mobile charge carriers. Absorption occurs when radiation interacts with

electrical or magnetic dipoles of shielding material and the absorbed radiation turns into heat energy.

Both reflection and absorption of shielding material depends on the high electrical conductivity. Thus,

EMI shield has to be thick enough to dissipate more radiation [10-14].


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Figure (1): EMI Shielding Mechanism.

There comes another term into spotlight, EMI shielding efficiency (EMI SE). EMI SE is the ability of

material to attenuate the EM wave strength, defined as the logarithm of incoming power (Pi) to

transmitted power (Pt) in the units of decibels (dB). The total EMI SE (SET) is the function of reflection

(SER) and absorption (SEA) and can be written as -

SET = SER + SEA……………… (1)

where the coefficient of reflection (R), transmission (T) and absorption (A) were calculated using S

parameters S11 and S21 or S12 and S22 according to following equations [9] :

The commonly used nanofillers are metal such as iron, aluminum, silver, carbon and its various forms,

graphenes, metal oxides, composite of different nanofillers etc. Conducting materials are preferred over

semiconducting materials due to low resistivity. It is reported that ferric oxide (Fe2O3)/reduced graphene

oxide (rGO)/polydimethylsiloxane (PDMS) composite with 10 mm thickness can be an effective EMI

shield with efficiency of 35.83 dB which can be reduced to 23.69 dB for a 2 mm thickness at 10 wt. %

of Fe2O3 loading [9]. Adapting same methodology, PVC/PMMA/GeO2 nanocomposite film at 10 wt. %

of GeO2 loading was studied for shielding efficiency at X-band, having efficiency of 17.14 dB. When

the same composite film was studied in the Ku-band (12 - 18 GHz), the maximum shielding efficiency

of 15.423 dB was observed [16]. Iron oxide included 30 wt. % NiFe2O4 nanoparticles filled BaTiO3

ceramics has been reported to have shown EMI shielding efficiency of more than 34 dB in the whole X-

band [17]. Similarly, EMI SE of PS/TGO/Fe3O4 hybrid nanocomposite showed more than 30 dB in X-

…………….. (3)

…………….. (2)


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band due to the reflectance loss by magnetic particles and absorption by TGO in polystyrene composites.

[18]. Presence of magnetic Co/Ni particles with SWCNTs shield of thickness 1.5 mm resulted in good

shielding efficiency of 24 dB in Ku-band [19]. It is also reported that the nanocomposite derived from

polypyrrole and magnetic Co nanoparticles with film thickness of 2 mm showed EMI shielding

efficiency (SE) of 33 dB in the Ku-band. It is therefore appropriate to consider that magnetic particles in

nanocomposite films may be better suited for shielding effectiveness and the efficiency can be altered by

changing the particles concentration, nature and film thickness [20]. In comparison of hybrid

nanocomposite films with magnetic and non-magnetic metal or metal oxides, the magnetic ones are

reported to give maximum shielding efficiency [21]. It is therefore possible to conclude that magnetic

nanoparticles especially iron oxide show significant increment and better shielding performance in any

hybrid or nanocomposite film combinations. The effort in this article is presented for simplicity of nano-

particle synthesis as well as developing its composite with PVA with mild shielding efficiency.

2. Experimental procedure:

2.1 Materials and methods:

Synthesis of Iron Oxide NPs: Ferrous sulphate heptahydrate (FeSO4.7H2O) from SRL chemicals, Ferric

chloride (FeCl3) from Sigma-Aldrich, Ammonia solution (NH3)aq from SRL chemicals, Coconut Oil

from local market, Ethanol (absolute) from Analytical Reagents, N-Heptane (95 %) from SRL

chemicals, Acetone (CH3)2CO from Fischer Scientific, De-ionized water (18.2 MΩ resistivity). For

casting of EMI shield: γ-Iron Oxide NPs, Polyvinylalcohol (MW 125000, degree of polymerization 1700

- 1800) from Kemphasol, Chloroform HPLC grade ( > 99.5 %) from Sigma-Aldrich, were used. Fourier

Transform-Infrared spectrometer in the wavenumber range of 400 - 4000 cm-1 (Perkin Elmer Spectrum-

Two) was used to obtain transmittance spectra and Ultraviolet-Visible spectrophotometer (Specord 210)

was used to measure the absorbance and band gap in the wavelength range of 200-800 nm, X-Ray

patterns were measured on Bruker D8 advance XRD at wavelength 1.54 Å along with scanning speed of

2˚ min-1, surface morphology and % composition of elements were obtained from Scanning Electron

Microscopy (ZEISS Gemini SEM) and Energy Dispersive X-Ray (EDX), particle size distribution

profile was obtained using Particle Size Analyzer (PSA) (NanophoxSympa TC) and Vibrating Sample

Magnetometer (VSM) was used for magnetization property of γ-Fe2O3 NPs at room temperature

(LakeShore Vibrating Sample Magnetometer Model 7404). Shielding effectiveness or efficiency of γ-

Fe2O3 NPs doped PVA films in X-band and Ku-band were measured using Vector Network Analyzer

(PNA Network Analyzer N5222A) at room temperature.

2.2 Synthesis of nano-particles and film preparation:

2.2.1 Synthesis of Iron oxide nanoparticles: Fe2O3 NPs were made by co-precipitation method with

FeCl3 and FeSO4 in the ratio (3:2) in de-ionized water taken in two-neck round bottom flask. The

solution mixture was heated at 80 - 90˚C under nitrogen atmosphere with continuous stirring

throughout. Few drops of coconut oil were added as surfactant/capping agent before the addition of

reducing agent. Ammonia solution was added as reducing agent until the pH of solution mixture reached

the range of 9 - 14 or basic. The reaction was carried out for 4 - 5 hours at the same condition followed

by cooling. The reaction was quenched with n-heptane and ethanol. The precipitate was centrifuged and

washed with ethanol and acetone. The separated particles were dried out in hot air oven at 50˚C

overnight. The particles were crushed into fine powder and stored.

2.2.2 Casting of EMI shield: EMI shields of three different particle loading were made by

impregnating with Fe2O3 NPs in poly vinyl alcohol (PVA) in 1:0.1; 1:0.5 and 1:1 w/w ratio. Typically,

1.0 gm of PVA was taken in the beaker containing a few mL of chloroform and heated the mixture to

form clear solution. 0.1 gm of γ-Fe2O3 nanoparticle was separately taken in few mL of chloroform or


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ethanol and sonicated for dispersion and then added to PVA solution (A10). The completely blended

solution was poured onto a mould and was allowed to dry until the chloroform evaporates and was left

overnight, leaving a film of Fe2O3/PVA nanocomposite. The final dried film was peeled out of the

mould. Similar procedure was practiced for preparation of 1:0.5 (A50) and 1:1 (A100) film. Another

film was also made with 1:1w/w of uncapped γ-Fe2O3 NPs and PVA (A00) to study the EMI shielding

effectiveness for comparison (Figure 2B).

3. Results and discussions:

Synthesis of iron oxide nano-particles in the present work was done using green surfactant as per the

reported protocol [8]. According to the possible steps for synthesis, the salts were taken in a beaker and

mixed with a few drops of coconut oil for controlling the particle growth and nucleation process. The oil

rich in carboxylic groups will offer effective surface capping to the particles. It is well documented that

coconut oil containing a several fatty acids and the main component is lauric acid. Thus it is believed

that iron lauriate or other such complex will be forming as intermediate complex during the synthesis

which eventually breaks down to generate iron oxide capped with fatty acid. The overall formation of

the nano-particles and their loading in polymer for casting the film is presented in Figure 2A and 2B.

The as-prepared nano-particles were characterized by various spectroscopic tools before using them for

casting the film.

The absorbance and band gap for the samples are obtained from UV-Visible spectroscopy measurement

in the range of 300 - 800 nm (figure 3). The maximum absorption observed was approximately between

575 - 580 nm and the corresponding band gap was estimated to be in the range of 2.1 - 2.2 eV. As a

result of small sized particle formation, blue shift with respect to bulk band gap energy value (~ 2 eV),

was observed. The band-gap of iron oxide may vary depending on the phases (α, β, and γ). The presence

of capping agent resulted in excellent size control of particle. The consistency in absorption value

indicated that iron oxide NPs with controlled size homogeneous size distribution with broad absorption

profile was formed in all preparations.

Figure (2): A) Synthesis of Fe2O3 NPs and B) Casting of EMI Shield/Film.


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Figure (3): UV-Visible spectra of as-prepared Fe2O3 NPs.

FTIR spectrum (Figure 4a) showed broad peaks at 550 cm-1, 1100 cm-1 and 1434 cm-1 for stretching

vibrations of metal-oxygen (Fe-O) and matched well with literature reports however, presence of other

phases may not be ruled out [8]. The spectra of Fe2O3 NPs and PVA exhibit a combination of various

bonds, deformation, stretching and vibrations. The dominance of Fe-O bond is observed from

characteristic peak at about 600 cm-1. The other low intensity peaks are considered due to C-O stretching

and –CH2 deformation, -C-OH stretching and γ(C-H) vibration respectively.

Figure (4): (a) Typical FT-IR Spectra of Fe2O3 NP sample (b) 1:1 w/w loading in PVA (A100).


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The peaks between 2100 cm-1 to 2400 cm-1 attributes to –CH3-CH2 stretching and C-H stretching

respectively. These distinct peaks show the intermolecular relation between polymers and nanofillers in

the nanocomposite films. Typically all composite films showed similar IR pattern, but since sample A3

was loaded to PVA and various loading of the samples were analyzed by IR therefore IR of A3 and of

1:1 composite film is presented in Figure (4).

The characteristic broad peaks observed from the XRD plot matches (figure 5) with the literature values

of γ-Fe2O3 NPs in the samples [8, 15]. The peaks in the range of 2ϴ (degree) of 30, 35, 43, 57, 63 was

observed due to diffraction from 211, 220, 311, 400, 511 and 440 crystal planes of γ-Fe2O3. The average

crystallite size (D) was estimated by Scherrer’s equation. The D values obtained for 311 crystal plan

about 15 nm. The crystal structure, the crystallite size (D) and interplanar spacing (d) matched well with

the literature values [15]. A typical XRD pattern of is presented below:

Figure (5): XRD pattern of γ-Fe2O3 NPs.

Figure (6) shows the SEM images of one of the samples. It is observed that the iron oxide NPs are

spherical in shape albeit not without agglomeration. The formation of clusters of controlled size can be a

because of the presence of capping agent in the reaction. The EDX/EDS studies are conducted to

investigate the composition of each element in the sample. For a typical sample A3 (Figure 6), the

elemental composition obtained showed strong peaks of Fe and O. The weight composition of Fe and O

is obtained as 58.59 % and 31.69 % respectively. The remaining weight % is observed due to the

presence of C and N in from the capping agent and reducing agent. (Table 1).

The particle size distribution analysis was done to understand the distribution of the particles in a given

sample after dispersion in a suitable solvent. Since the particles can undergo cluster formation, unit

dispersion is ensured before analyzing particle size. Figure (7) shows the particle size distribution

profile. The distribution varied in the range of about 5 - 25 nm and the average size obtained for the

particle was about 10 nm indicating reasonable narrow distribution.


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Figure (6): SEM images and EDX of Fe2O3 NPs

Table (1): Element composition (%) from EDX.

Figure (7): Particle size distribution profile of as-prepared Fe2O3 NPs.

The magnetization curve was plotted within -1T and +1T emu/g and the magnetic behavior of particles

can be studied from this plot. The increment of the plot is showing about the paramagnetic behavior of

sample in the positive direction. The magnetic flux density was observed to be increasing with magnetic

field and gets saturated beyond the field intensity of 5500 G and the saturation was obtained in the range

EDX Data with Carbon and Nitrogen

Elements Fe O C N

Weight % 58.59 31.69 7.89 1.84

Atomic % 27.48 51.88 17.20 3.44

EDX Data without Carbon and Nitrogen

Elements Fe O

Weight % 69.78 30.22

Atomic % 39.82 60.18


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of 0.9 to 1 emu/g. The curve indicate the low coercivity and moderate saturation magnetization of the

nano-particles (Figure 8).

The results indicate near superparamagnetic nature of the nano-particles, thus making these

nanoparticles useful candidates for EMI shielding application because of possibility of relaxation

mechanism occurring at higher frequencies. It has been studied that the magnetization saturation

decreases when the magnetic particles are incorporated with non-magnetic polymer matrix and thus

shielding efficiency has also been documented [10]. The purpose of magnetic particles in shielding

application is its property to absorb or reflect magnetic part of the EM wave. It is also reported that

magnetic behavior can offer corrosion resistance [22].

Figure (8): Magnetization curve of nanoparticle.

The VNA analysis conducted in X-band and Ku-band giving better information about the dielectric

properties and shielding effectiveness of any nanocomposite films. The X-band of frequency from 8.2

to 12.4 GHz is selected because most of the electronic components emit EM radiations in this frequency

range and satellite communications interference are coming in Ku-band of frequency 12.4 to 18 GHz. In

the current study, the EMI shield shows poor efficiency in lower frequency but as the frequency

enhances, the effectiveness also enhanced. Since the total shielding efficiency is the cumulative effect of

reflection and absorption, the dominancy of absorption phenomena of EM waves is studied from the

above plotted graph. It is observed that in the shield/film enriched with NPs, shielding effectiveness

increases with increase in particle concentration. The EMI shielding effectiveness (EMI SE) of capped

Fe2O3/PVA nanocomposite films of ratio (w/w) 0.1:1; 0.5:1 and 1:1 in X-band is obtained as -0.38, -

0.49 and -1.94 dB respectively (Figure 9a-b). The uncapped Fe2O3/PVA nanocomposite films showed

moderate shielding efficiency of -3.45 dB in the X-band for film with 1:1 ratio.


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Figure (9): EMI shielding performance in X-band of nanocomposite films

(a) A00 (b) A10 (c) A50 (d) A100.

However, the EMI shielding effectiveness of nanocomposite films A00, A10, A50 and A100 in Ku-band

is obtained as -10.39, -7.70, -9.29 and -13.06 dB respectively (Figure 10a-d). During the dipole

polarization, large amount of energy is dissipated and this paves the way to maximum absorption of EM

waves under varying EM field. Thus capped Fe2O3 NPs containing PVA nanocomposite films with 1:1

ratio can be considered as an eco-friendly efficient way to protect electronic gadgets from EM pollution.

The data are presented in Table (2) and their graphical reflection is presented in Figure (11).


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Figure (10): EMI shielding performance of films in Ku-band (a) A00 (b) A10 (c) A50 (d) A100.

Table (2): Compilation of EMI SE of nanocomposite films in X-band and Ku-band.

Frequency

Bands

Wt. % of capped Fe2O3 in PVA Wt. % of uncapped Fe2O3 in PVA

A10 (1:0.1) A50(1:0.5) A100(1:1) A00(1:1)

Max Shielding effectiveness of sample films

A10 A50 A100 A00

X-band -0.38 -0.49 -1.94 -3.45

Ku-band -7.70 -9.29 -13.06 -10.39


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Figure (11): Graphical presentation of EMI shielding performance of various nanocomposite films.

4. Conclusions: Stable super paramagnetic Fe2O3 NPs are synthesized using coconut oil under nitrogen

atmosphere. The experiments were carried out at moderate temperature of about 90 °C, in alkaline pH

employing ammonia solution. Formation of Fe2O3 NPs was confirmed by characterization techniques

like UV-Visible, FT-IR, XRD, SEM/EDX. Polymer nanocomposite films of less than 1 mm were

fabricated with various ratios viz; 1:0.1, 1:0.5 and 1:1 w/w (PVA: NPs) for capped Fe2O3 NPs and 1:1

w/w for uncapped γ-Fe2O3 NPs and EMI shielding efficiency was measured using VNA instrument in

X-band and Ku-band. As the concentration of nano-particle increases, the absorption of EM waves

enhances and thereby increases the shielding efficiency. Uncapped Fe2O3 NPs loaded nanocomposite

shield showed maximum efficiency of -3.45 dB however, the capped Fe2O3 NPs containing films

showed efficiency of only 1.94 dB in X-band. EMI SE in Ku-band was maximum for capped Fe2O3 NPs

based nanocomposite film with value of -13.06 dB which was higher than uncapped Fe2O3 NPs

containing nanocomposite film with the efficiency of -10.39 dB.

Acknowledgments: Authors are thankful to Vice-chancellor DIAT, Pune for support and permission.

PKK thanks Ms. Priyanka for assistance during revision.

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